The Ichcahuipilli was a common form of padded cotton armor utilized throughout Mesoamerica.
The Ichcahuipilli was an extremely common piece of battle dress in Mesoamerica. The Anonymous Conqueror described it as being very strong, going on to say that it was “one or two fingers thick”of padded cotton.
Padded cotton or linen was an extremely common form of light armor throughout history in nearly every part of the world. Despite its flexibility, it is extremely durable and helps protect the user from blunt force, stabbing, and slashing. A Discovery Channel documentary demonstrated perfectly how a thick linen cuirass can successfully stop an arrow fired from a powerful English warbow (https://www.youtube.com/watch?v=CULmGfvYlso).
Aztec cuirasses used cotton rather than linen, but its protective capability was likely comparable. In addition, the padded cotton would have been better able to stop obsidian based weaponry as opposed to Old World steel weapons. Ross Hassig notes that, in times of need, less protective Agave fibers could have been used in place of cotton (i.e. in Michoacan where it is difficult to grow cotton).
Despite the fact that many soldiers in the Aztec army probably fought while wearing little more than a loin cloth, warriors of status are almost always depicted wearing the Ichcahuipilli. Curiously, warriors who appear to have no distinction are also occasionally depicted with padded armor. The Codex Mendoza and the Lienzo de Tlaxcala depict archers with padded jackets, despite the fact that archers were probably entirely composed of drafted levies and were never expected to achieve any sort of military accomplishments.
Although the Aztec elite despised the bow, many neighboring tribes and polities adopted it fully, meaning that enemy states might have equipped their archers better than their Aztec counterparts. However, it is unclear whether some of the depictions show Aztec archers in Ichcahuipilli, or the soldiers of some other polity.
Although it is commonly cited that the Ichcahuipilli was dipped in saltwater and dried in order to harden for increased protection, Ross Hassig (and others) argue that this is a myth that is probably the result of a mistranslation. The success of the Ichcahuipilli in Mesoamerican warfare was in no small part due to the fact that it was flexible and allowed the user to move unhindered. Hardening it would drastically reduce many of the advantages that it provided. In any case, only one (probably erroneous) source suggests that this was the case.